The West Valley Highway was built in the early 1910s, after an alignment on the west side of the White (now Green) River Valley was chosen by county engineers in 1909. SR 181 was formed during the
1964 state highway renumbering as a highway following the
Green and
Duwamish rivers from
SR 18 in
Auburn through
Kent and
Tukwila to
SR 900 in
Seattle. The route was originally signed in 1937 as SSH 5M from
PSH 5 in Auburn to
U.S. Route 99 (US 99) and
PSH 1, and in 1931 as a branch of PSH 1 from SSH 5M to
PSH 2. SSH 5M was renumbered to SSH 2M in 1959 and the southern terminus of the highway was moved to a new branch of PSH 2. SSH 2M and the PSH 1 branch were combined into SR 181 in 1964, and was truncated to newly created
SR 599 in 1971. The northern terminus was moved to its current location, an interchange with
I-405 in 1979, and the southern terminus was moved to Kent in 1991. No major revisions to the route of the highway have occurred since 1991 and the highway is no longer connected to parent route SR 18. ==Major intersections==